1. Field of the Invention
The present invention relates to a technology of an image generating apparatus and an image generating method for fetching a motion picture at a variable frame rate and generating compression-encoded image data.
2. Related Background Art
There are digital still cameras and digital video cameras as devices capable of photographing an image and recording and reproducing the image. Recent image pickup elements employ an increased number of pixels due to an advanced manufacturing technology, thereby the quality of images photographed by not only digital still cameras but also by digital video cameras using the image pickup elements is greatly improved (resolution of the images is increased).
When an image having a high resolution is generated using the cameras described above, although the photographing capability, image compression capability, recording capability, and the like of them are greatly influenced due to a large amount of image data, the operating conditions of the cameras are optimally set according to the respective cameras and the modes thereof by restricting a processing time and an amount of processing data.
For example, there is a digital video camera which records pickup data of 480×720 pixels in NTSC or 576×720 pixels in PAL (corresponding to 640×480 square pixels in VGA) at a rate of 30 frames/sec in a motion picture recording mode which assumes that a motion picture is output to a TV monitor, and which records pickup data of 1,230,000 pixels (corresponding to square pixels 1280×960 pixels in SXVGA) in a still image recording mode.
Further, there is a popular type digital still camera which records a still image of 3,000,000 pixels (corresponding to 2048×1536 pixels in QXGA) or more and also continuously photographs a motion picture at a rate of about 2 frames/sec.
In general, however, even a digital video camera and a digital still camera having an image pickup element including a large number of pixels can only photograph and record an image in a fixed pixel range that is uniquely determined according to a photographing mode and an image size set by a user, and it is difficult to optionally obtain a photographed image having a high resolution of, in particular, a motion picture.
In contrast, for the digital video cameras, users have a desire for executing photographing at a high rate, in addition to a desire for increasing the number of pixels. For example, there is the need for changing a frame rate to a high rate side even while a motion picture is being photographed. However, the increase in the frame rate not only increases a load on respective processing circuits but also causes a possibility that a reproduction unit or a display unit cannot cope with a motion picture photographed at a changed frame rate when the picture is reproduced. Thus, these problems must be overcome when a digital video camera that fulfills the need is actually manufactured as a product. In view of the above problems, there is conceived a motion picture photographing system that makes it possible to record a motion picture at a different frame rate by controlling the system by a host computer connected to a camera and the like as disclosed in U.S. Pat. Nos. 5,640,202 and 5,786,851, for example.
However, actually employed as the frame rate at which a motion picture is recorded is a predetermined frame rate set according to the system of a TV monitor and the like or a frame rate set under the control of a host computer as disclosed in the above US patents, and it cannot be attained that a user manipulating a video camera can optionally change the frame rate.
As described above, in the conventional technologies, the resolution of a photographed image is determined according to a preset image size. Further, since the number of frames to be processed per second, and the like are uniquely determined by the read-out time of an image pickup element, the processing time of an image compression unit, and the data transfer rate of a recording unit and the like, in the conventional system, a resolution and a frame rate cannot be selected on a camera side.